This application claims priority to Korean Patent Application No. 2004-9243, filed on Feb. 12, 2004, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates generally to amplifier circuits, and more particularly to a regulated cascode amplifier with a simple small-sized feed-back amplifier.
2. Description of the Related Art
Generally, a cascode amplifier is used as a gain stage of differential amplifiers for providing large output impedance and large voltage gain. FIG. 1 shows a circuit diagram of a conventional cascode amplifier 100. The cascode amplifier 100 includes a current source IB1, a first NMOSFET (N-channel metal oxide semiconductor field effect transistor) 101, and a second NMOSFET 103.
The cascode amplifier 100 amplifies an input voltage Vin to generate an output voltage Vout. The input voltage Vin is applied on a gate of the first NMOSFET 101, and a source of the first NMOSFET 101 is coupled to a ground node (GND). A bias voltage VB1 is applied on a gate of the second NMOSFET 103 such that a constant current flows through the second NMOSFET 103. The NMOSFETs 101 and 103 are stacked with a source of the second NMOSFET 103 being coupled to a drain of the first NMOSFET 101.
A drain of the second NMOSFET 103 is coupled to the current source IB1 at a node that generates the output voltage Vout. An output impedance Rout of the cascode amplifier 100 at the drain of the second NMOSFET 103 is expressed as follows:Rout=rds2+rds1(1+gm2 rds2)
rds1 is a resistance between the drain and the source of the first NMOSFET 101, rds2 is a resistance between the source and the drain of the second NMOSFET 103, and gm2 is a transconductance of the second NSMOFET 103.
The voltage gain Av of the cascode amplifier 100 is expressed as follows:Av=Vout/Vin≈gm1rds1(1+gm2rds2)
gm1 is a transconductance of the first NSMOFET 101. Such equations illustrate that the output impedance Rout and the voltage gain Av are relatively large for the cascode amplifier 100.
FIG. 2 shows a circuit diagram of a conventional regulated cascode amplifier 200, as disclosed in IEEE JSSC SC-22 (pp. 287–294) by E. Sackinger dated 1987. The conventional regulated cascode amplifier 200 includes a main cascode amplifier 210 and a feedback amplifier 220.
The regulated cascode amplifier 200 amplifies an input voltage Vin to generate an output voltage Vout. The main cascode amplifier 210 includes a current source IB2, a first NMOSFET (N-channel metal oxide semiconductor field effect transistor) 211, and a second NMOSFET 213.
The input voltage Vin is applied on a gate of the first NMOSFET 211, and a source of the first NMOSFET 101 is coupled to a ground node (GND). The NMOSFETs 211 and 213 are stacked with a source of the second NMOSFET 213 being coupled to a drain of the first NMOSFET 211. A drain of the second NMOSFET 213 is coupled to the current source IB2 at a node that generates the output voltage Vout.
The feedback amplifier 220 includes a third NMOSFET 221 and a PMOSFET (P-channel metal oxide semiconductor field effect transistor) 223. A bias voltage VB2 is applied on a gate of the PMOSFET 223 such that a constant current flows through the PMOSFET 223.
An output impedance Rout of the regulated cascode amplifier 200 at the drain of the second NMOSFET 213 is expressed as follows:Rout=rds2+rds1[1+gm2rds2(1+gm3(rds3∥rds4))],                rds1, rds2, rds3, and rds4 are each a respective resistance between the respective source and the respective drain of the first, second, and third NMOSFETs 211, 213, and 221, and the PMSOFET 223, respectively. In addition, (rds3∥rds4) is an equivalent resistance of rds3 and rds4 in parallel. Furthermore, gm2 and gm3 are each the transconductance of the second and third NMOSFETs 213 and 221, respectively.        
The voltage gain Av of the regulated cascode amplifier 200 is expressed as follows:Av=Vout/Vin≈gm1rds1[1+gm2rds2(1+gm3(rds3∥rds4))],
gm1 is a transconductance of the first NSMOFET 211.
Such equations illustrate that the output impedance Rout and the voltage gain Av are relatively large for the regulated cascode amplifier 200. However, the conventional regulated cascode amplifier 200 has large chip area because a bias circuit is needed for generating the bias voltage VB2.